During pregnancy many hormonal changes occur to insure normal development of the fetus and preparation of the mother for lactation. Some of these hormones are present in the nonpregnant organism, while others are only present during pregnancy. Prolactin from the pituitary gland has a much different secretion pattern during pregnancy, and in the human, is also secreted by the decidual portion of the placenta. In the rat, during the first half of pregnancy there are two daily surges of prolactin. These are important in maintaining the secretion of progesterone by the corpora lutea. These surges are absent during the second half of pregnancy, and a hormone from the placenta called placental lactogen (PL) maintains the corpora lutea. Prolactin and PL are similar structurally, and possess some similar functional properties as well. The long term objectives of this research is to determine what factors control prolactin secretion during pregnancy, and to determine the interrelationship between prolactin and PL in the rat. The specific aims of this project are: (1) Determine the role of dopamine and thyrotropin releasing hormone (TRH) on initiation and maintenance of prolactin surges during pregnancy; (2) Determine the function of opiatergic neurons in regulating prolactin secretion; (3) Determine which components of the conceptus terminate prolactin surges at midpregnancy; (4) Determine the location of PL and its receptors in the brain; (5) Determine what neural factors controlling prolactin that PL affects; (6) Determine the influence of PL in inhibition of gonadotropin secretion during pregnancy. A variety of in vivo and in vitro approaches will be taken to achieve these goals. Hypophysial portal blood will be analyzed for DA and TRH. Pushpull perfusion of the arcuate-median eminence area and subsequent analysis for DA and TRH will utilize unrestrained, conscious rats. Extensive studies will be done with beta- endorphin, determining its site of action and via what mechanism it regulates prolactin. Measurement of hormone secretion will be done by radioimmunoassay and bioassay using a cell culture technique. This research will reveal what factors are important in controlling prolactin, an essential hormone for pregnancy.